1. Field of the Invention
The present invention relates to an electroluminescent device (EL device) that is used, for example, in the emissive displays of instruments or in the displays of various information terminal equipment.
2. Description of the Related Art
Conventionally, EL devices utilize a phenomenon in which light is emitted when an electrical field is applied to a fluorescent material such as zinc sulfide (ZnS). They are attracting attention as devices that compose self-illuminating type flat displays.
FIG. 5 is a schematic diagram indicating the typical cross-sectional structure of conventional EL device 10.
EL device 10 is formed by sequentially laminating a first electrode 2, composed of an optically transparent ITO (indium tin oxide) film, a first insulating layer 3, composed of tantalic pentoxide (Ta.sub.2 O.sub.5) and so forth, a phosphor layer 4, a second insulating layer 5, and a second electrode 6, composed of an ITO film, on an insulated substrate in the form of glass substrate 1.
ITO films are transparent and conductive films wherein tin (Sn) is doped onto indium oxide (In.sub.2 O.sub.5), and have been widely used for transparent electrodes due to their low resistivity.
Examples of phosphor layer 4 include those in which zinc sulfide (ZnS) is used for the host, which is doped with manganese (Mn) or terbium (Tb) as the luminescent center, as well as those in which strontium sulfide (SrS) is used for the host, which is doped with cerium (Ce) as the luminescent center.
The emitted colors of EL devices are determined by the type of dopant in the zinc sulfide. For example, in the case of doping with manganese (Mn) as the luminescent center, yellow-orange light is emitted, while in the case of doping with terbium (Tb), green light is emitted. In addition, in the case strontium sulfide (SrS) is doped with cerium (Ce) as the luminescent center, blue-green light is emitted.
In order to realize full-color EL displays, it is necessary to form an EL phosphor layer that emits red, green and blue light. Examples of phosphor layer materials of EL devices that emit blue color typically include those in which strontium sulfide (SrS) is doped with cerium (Ce) as the luminescent center.
However, in the case of using this phosphor layer material, since this material inherently emits blue-green light, in order to obtain only blue light, it is necessary to use a filter that cuts the green component of the emission spectrum as is indicated on p. 29-32 of Society of Information Display (SID) 86 Digest.
In addition, as is indicated on, for example, p. 761-764 of Society of Information Display (SID) 93 Digest, EL devices, wherein alkaline earth metal thiogallates (MGa.sub.2 S.sub.4 M.dbd.Ca, Sr, Ba) are used for the host and then doped with cerium (Ce) as the luminescent center, are known to allow emission of blue light to be obtained without using a filter.
However, since the phosphor layer of alkaline earth metal thiogallates doped with cerium (MGa.sub.2 S.sub.4 : Ce, M.dbd.Ca, Sr, Ba) are formed by sputtering, a thin film having good crystallinity were unable to be obtained in the past. Consequently, heat treatment is necessary at a temperature of at least 650.degree. C. at which softening of the glass substrate occurs in order to improve crystallinity of the phosphor layer following formation of the phosphor layer thin film.
However, when heat treatment at this temperature is performed, warping of the glass substrate is unable to be avoided, thus creating a major problem in terms of the structure of the blue EL device.
In addition, in blue EL devices on which this heat treatment has been performed, such devices conventionally have not been able to demonstrate a practically sufficient luminance. Although there are many factors regarding the relationship the crystallinity of the phosphor layer thin film and EL luminance that remain unclear, this is considered to be caused by being unable to obtain a means for obtaining high luminance.